Refractory bricks are subject to extreme temperature conditions, frequent thermal cycling, and corrosive attack. For many years, magnesia-chrome refractories were used as liners for rotary lime kilns, cement kilns, or similar uses, because they displayed properties that met the demands of the refractory environment. Magnesia-chrome bricks exhibited long wear, high melting temperatures, low thermal conductivity, good hot strength, and good thermal shock resistance. Despite these many advantages, magnesia-chrome based refractory linings also suffered a serious disadvantage. The chrome within the magnesia-chrome brick reacted with the lime and alkali in the refractory environment to form hexavalent chrome. Hexavalent chrome was categorized as a toxic material requiring special disposal measures. Thus, the use of refractory linings fabricated from magnesia-chrome resulted in costly disposal and remediation procedures at the end of their use. These issues forced manufacturers to seek other alternative materials for refractory liners.
For the past 20 years, magnesia-spinel brick became widely used in the art as an alternative for magnesia-chrome brick liners. The magnesia, MgO, present in the magnesia-spinel brick makes the brick basic so that it can withstand the highly basic environment of the rotary cement kiln. Magnesia, however, exhibits high thermal expansion, high thermal conductivity, and poor thermal shock resistance. To alleviate these problems, spinet, MgAl.sub.2 O.sub.4, is added into the refractory mix. Spinel has lower thermal conductivity and lower thermal expansion than magnesia. When spinel is added to the magnesia, the magnesia is essentially diluted and the thermal expansion and thermal conductivity of the refractory is thereby reduced. Thermal shock resistance is also improved because of the large difference in thermal expansion between the magnesia and spinet. This thermal expansion mismatch creates microcracks in the brick, which improves thermal shock resistance. Thus, the addition of spinet to the magnesia product creates a refractory that is basic in chemistry and has good thermal shock resistance, low thermal conductivity, and low thermal expansion.
Over the 20-year period of magnesia-spinel brick use, several distinct evolutionary steps in composition formulation can be identified. These distinct steps are generally referred to as `generations` within the refractory industry. First generation magnesia-spinel brick are based on the use of magnesia and in situ spinet. The in situ spinel is formed by adding small amounts of relatively coarse alumina to the brick to form spinet during firing. Second generation magnesia-spinel brick are characterized by the use of preformed spinet in the brick. The use of preformed spinel allowed more spinel to be added to the brick, which improved thermal shock resistance and lowered thermal expansion and conductivity. The preformed spinet grain added to the composition can be either sintered or fused. Third generation magnesia-spinel brick are characterized by the use of preformed spinet in combination with a very fine alumina addition. The fine alumina reacts with the magnesia fines in the mix during firing to form in situ spinet, commonly referred to as a spinet matrix. The addition of fine alumina to the refractory composition lowers porosity, permeability, and improves intermediate temperature strength at temperatures of about 2300.degree. F.
Unfortunately, third generation magnesia-spinel brick are not an ideal replacement for the magnesia-chrome bricks because they do not perform well under high thermal loading conditions. First, the third generation magnesia-spinel brick exhibit low strength performance, below 300 psi, at high temperatures (greater than 2500.degree. F.). Second, the spinel portion of the brick, particularly in the matrix, is susceptible to corrosion when exposed to high lime slags or clinkers within the refractory environment. As a result, the service performance of these bricks under high thermal loading conditions is limited and more frequent replacement of these liners is warranted.
The spinel is generally the "weak link" in all types of magnesia-spinel brick used in high temperature zones of the rotary kiln. The alumina within the spinel readily reacts with lime to form calcium-aluminates. Some of these calcium-aluminate phases tend to have lower melting temperatures, below 2500.degree. F., than the temperatures of the refractory environment. Additionally, if silica is present, as is often the case in a refractory melt, calcium-aluminum-silicate phases can form which also have lower melting points than the temperatures of the refractory environment. Therefore, magnesia-spinel bricks do not perform well in kilns that operate at fairly high temperatures or temperatures above about 2500.degree. F. These bricks weaken as certain phases soften and melt into liquid phases within the brick above about 2500.degree. F. Lastly, these liquid phases can cause densification at the hot face, which can eventually lead to spalling.
Accordingly, there exists a need in the art to develop a refractory brick with improved strength, particularly above 300 psi, when used at temperatures exceeding 2700.degree. F., and also to develop a refractory brick that has improved resistance to lime thereby increasing its service life.